1. Field of the Invention
The invention relates to the field of roasting furnaces, and more particularly to furnaces which have multiple, vertically stacked hearths.
2. Description of the Art
Multiple hearth furnaces have been in use for many years, primarily in the mineral processing industry for purposes such as ore roasting, drying, and calcining (e.g., the production of lime from limestone). Commercial furnaces include several variations, such as the Herreshoff, Nichols, Wedge, Skinner, and other designs. In general, these devices are fabricated using a metal exterior structure, lined with refractory and having refractory hearths which define the limits of roasting chambers. Heat is applied by fuel burners, which introduce combustion gases directly into one or more of the vertically stacked chambers. Heat is not applied evenly, resulting in higher- and lower-temperature areas above given hearths, and uneven roasting, drying, or calcining conditions.
Even more serious, however, are disposal problems with gases from the furnaces. For example, the roasting of sulfide ores generally requires a large amount of air introduction, both for maintaining a sufficient oxygen concentration to permit sulfide sulfur to form sulfur oxides, and for cooling to minimize "hot spots" in the furnace. As a consequence, the sulfur oxide effluent not only contains combustion products from the burners and dust (composed of furnace feed and roasted products), but is diluted with air. It is no longer environmentally acceptable to simply release sulfur oxides into the atmosphere, but sulfur oxide recovery is made more difficult by the contaminants. Indirectly heated furnaces are known, as taught by Leggo in U.S. Pat. No. 837,576. FIG. 42 of the patent shows a four-chamber furnace constructed of bricks, wherein heat is supplied beneath the chambers from burners located in "fire boxes" adjacent to the chambers. Separate systems are present for removing gases from the chambers and combustion gases.
The disadvantages of brick construction include diffuculty of making repairs without completely rebuilding large sections or all of a structure, a lack of versatility in forming desired shapes and configurations, permanence of the structure when changes are desired, poor heat transfer characteristics through the bricks, and contamination of materials passed through the furnce, due to abrasion or spalling of the bricks. Several of these disadvantages are also present in the modern multiple-hearth furnace, which utilizes refractory insulation and hearths inside the furnace.
Accordingly, it is an object of the present invention to provide a multiple hearth furnace which can be easily constructed, repaired, and operated.
It is a further object to provide a furnace having modular construction, permitting ease of modification for accomplishing various operations.
Another object is to provide a furnace which can be used to produce high-purity chemical products.
An additional object is to provide a furnace in which the inclusion of large quantities of dust in the gases discharged from the furnace will be avoided.
A still further object is to provide a furnace wherein the combustion gases can be discharged or utilized for other heating purposes, without treatment for pollution abatment.
Yet another object is to provide a furnace wherein gases from the heated chambers can be separately collected for treatment or recycling.
These and other important objects will become more apparent to those skilled in the art, from consideration of the following description of the invention.